This invention relates to rupture disk assemblies and, more particularly, but not by way of limitation, to rupture disk assemblies which operate at low pressures and use a vacuum support to prevent reverse rupture of the rupture disk.
A large variety of safety pressure relieving devices of the rupture disk type have been developed and used. Generally, these devices include a rupture disk supported between complementary supporting members or flanges which are connected to a relief connection in a vessel or system containing fluid pressure. When the fluid pressure within the vessel or system exceeds the design rupture pressure of the disk, the disk ruptures causing fluid pressure to be relieved from the vessel or system through the ruptured disk.
One material of which rupture disks are commonly made is carbon, or more precisely carbon graphite. Carbon rupture disks which have been impregnated with resins to make them gas-tight have been found to be advantageous in that they are economical to produce, have excellent chemical corrosion resistance, do not creep and fatigue as a result of pressure variations, and reliably rupture at their predetermined rupture pressure regardless of temperature variations.
Carbon rupture disks have been used in low pressure applications. If the carbon rupture disks are rated for burst pressures of approximately 22 psig or less, it is common practice to use a vacuum support to prevent reverse rupture of the disk. That is, for example, if the pressure on the inlet side of the rupture disk (within the equipment which the rupture disk is protecting against overpressure) drops to 0 psig, the atmospheric pressure (assuming the relief system is vented to atmosphere) exerted on the downstream or outlet side of the rupture disk will be approximately 15 psig and can cause reverse flexure and cracking or rupture of the disk, since carbon graphite is a brittle material. Therefore, the vacuum support is used to bolster the inlet side and prevent reverse flexure and rupture under vacuum or reverse pressure conditions.
U.S. Pat. No. 4,102,469 to Shegrud discloses a prior vacuum support. Referring to FIG. 3, Shegrud discloses a vacuum support 42 which fits into lined bores 44, 45 and has a plurality of passageways or channels 52 therethrough for communication of pressure between the pressure relief passageway 53 and the frangible diaphragm 43. A circumferential shoulder 54 on the vacuum support 42 rests upon the liner 47 at the step 46 and prevents the bottom 54 of the vacuum support from exerting unwanted pressure on the frangible diaphragm 43. A problem with the Shegrud vacuum support is that the vacuum support restricts the flow through the rupture disk when the rupture disk ruptures due to overpressure from the protected vessel. The circumferential shoulder 54 prevents the vacuum support from passing through the rupture disk and therefore the flow through the rupture disk and vacuum support is limited to the flow capacity of the channels 52 through the rupture disk.
U.S. Pat. No. 4,315,575 to Schwarz discloses another vacuum support for a burst protection device. Schwarz' vacuum support is formed of a multiplicity of graphite rings. The rings have a prismatic cross section with the prism surfaces inclined relative to the plane of the disk. Schwarz discloses that, if the protected vessel is operated at a reduced pressure, the individual rings are braced against each other and in the case of an overpressure, the rings are pushed out with the rupture disk. The Schwarz device is a relatively complex vacuum support in which molded graphite bodies are sawed, turned, or ground to create the rings. In such a device, precision tolerances are critical, as any relative slippage of the prismatic surfaces of the rings away from the rupture disk during an underpressure (or overpressure on the outlet face) will allow the pressure plate 3 and the brittle rupture disk to flex toward the inlet and crack or rupture. It is contemplated that this relative slippage of the prismatic rings towards the rupture disk during an overpressure creates the need for the pressure plate 3 to equalize pressure loading between the vacuum support and the rupture disk and to thereby prevent premature rupture of the rupture disk when the rings are subjected to positive pressure loading from the protected vessel. The inclined prism surfaces of the rings also create a problem in that either the larger end of the prismatic rings must be larger than the rupture zone of the rupture disk, as illustrated in the figure, in which case the outermost ring(s) appear to be too large to pass through the rupture zone and to restrict flow through the rupture disk; or, if the larger end of the prismatic rings were to be made the same size as the rupture zone, the smaller end of the inclined holding ring 6 would have to be smaller than the rupture zone and therefore restrict the flow passageway in order for the inclined surfaces of the ring 6 to brace the prism-shaped rings during a vacuum. Schwarz discloses that the prismatic rings may consist of at least two parts, but does not disclose or suggest how or why such parts would be made, interact, affect operation of the rings, etc.
Therefore, there is a need for a vacuum support which does not restrict flow through the rupture disk, i.e., that allows full flow through the ruptured disk without creating a flow restriction. There is a need for such a vacuum support which is simple and inexpensive to manufacture and in which precise machining tolerances of fitted or interlocking parts are not required. There is a need for such a vacuum support which eliminates the use of pressure plates to equalize pressure loading between the vacuum support and the rupture disk.